Lithium iron phosphate cathode materials, preparation methods and applications

Abstract

This invention discloses lithium iron phosphate (LFP) cathode materials, their preparation methods, and applications. The LFP cathode material includes a matrix and a carbon coating layer covering the matrix. The corrosion resistance ω of the LFP cathode material is 0%–40%, and ω = (βI1) / (αI0), where α is the mass fraction of iron (excluding divalent iron) in the LFP cathode material; β is the mass fraction of elemental iron in the LFP cathode material; I1 / I0 is related to the diffraction intensity of a specified crystal plane in the LFP cathode material's XRD pattern and the diffraction intensity of the corresponding crystal plane in the standard XRD card; the specified crystal plane is one of the (101), (112), (011), (010), and (001) crystal planes. In this application, the corrosion resistance ω is limited to a reasonable range, which is beneficial for balancing cycle performance and rate performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and more specifically, to lithium iron phosphate cathode material, preparation method and application. Background Technology

[0002] With the rapid development of lithium-ion battery technology, lithium iron phosphate (LiFePO4) has become one of the most widely used cathode materials in power batteries and energy storage systems due to its advantages such as high safety, long cycle life, low cost, and environmental friendliness. In practical applications, to improve the electronic conductivity and ion transport performance of lithium iron phosphate materials, carbon coating technology is usually used to form a conductive carbon layer on its surface to improve its electrochemical performance.

[0003] Carbon coating processes mainly include solid-state methods, sol-gel methods, hydrothermal methods, and spray drying methods. These processes involve introducing organic carbon sources (such as glucose, sucrose, citric acid, asphalt, etc.) onto the surface of precursors or finished materials, followed by high-temperature pyrolysis to form a uniform or partially covered carbon layer structure. This carbon layer effectively enhances electron conduction between particles and suppresses side reactions between active substances and the electrolyte, thereby improving the material's cycle stability and coulombic efficiency.

[0004] However, in actual preparation, the distribution of the carbon coating layer is often difficult to control precisely. On the one hand, uneven mixing of the carbon source or improper pyrolysis conditions can lead to inconsistent carbon coating thickness, resulting in locally thin or missing areas. This causes structural degradation of the material during long-term charge-discharge cycles, significantly reducing its cycle performance. On the other hand, excessive optimization of process parameters in pursuit of coating uniformity can lead to an overly dense or continuous carbon layer, hindering the rapid diffusion of lithium ions on the electrode material surface and thus limiting the material's rate performance. Therefore, ensuring good cycle performance while maintaining excellent rate performance has become a key challenge for current carbon-coated lithium iron phosphate cathode materials. Summary of the Invention

[0005] The purpose of this invention is to provide lithium iron phosphate cathode materials, preparation methods, and applications, which are beneficial for lithium-ion batteries to balance cycle performance and rate performance.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a lithium iron phosphate cathode material, the lithium iron phosphate cathode material comprising a matrix and a carbon coating layer covering the matrix, wherein the corrosion resistance ω of the lithium iron phosphate cathode material is 0%-40%, and ω=(βI1) / (αI0), wherein,

[0008] α represents the mass fraction of iron (excluding ferrous iron) in the lithium iron phosphate cathode material.

[0009] β represents the mass fraction of elemental iron in the lithium iron phosphate cathode material;

[0010] I0 is the standard diffraction intensity of a specified crystal plane in a lithium iron phosphate crystal as recorded on a standard XRD card.

[0011] I1=Iˋ 最强 I1ˋ / I 最强 Among them, Iˋ 最强 The strongest peak in the standard XRD pattern; I 最强 The XRD pattern of lithium iron phosphate cathode material with Iˋ 最强 The diffraction intensity of the corresponding crystal plane; I1ˋ is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of lithium iron phosphate cathode material;

[0012] The specified crystal plane is one of the (101), (112), (011), (010), and (001) crystal planes.

[0013] In an optional embodiment, the matrix is ​​lithium iron phosphate doped with titanium;

[0014] And / or, α is 0%-1.5%;

[0015] And / or, β is 0%-0.2%;

[0016] And / or, I1 / I0 is 1.2-3.

[0017] In an optional embodiment, the detection method of α includes: taking a lithium iron phosphate cathode material sample m0 = 5.0000 g ± 0.0010 g and immersing it in 10 ml of dilute sulfuric acid with a concentration of 0.01 mol / L at 18-30℃ for 10 min, and detecting the concentration of dissolved iron ions c0 g / ml by potentiometric titration, α = (c0 * 10 / m0) * 100%;

[0018] And / or, the detection method for β includes: taking a lithium iron phosphate cathode material sample m1 = 5.0000 g ± 0.0010 g and placing it in 10 ml of a 0.1 mol / L copper sulfate solution at 18-30℃ and reacting for 10 min, then using ICP to detect the iron ion concentration c1 g / ml in the solution, and β = (c1*10 / m1)*100%.

[0019] Secondly, the present invention provides a method for preparing the lithium iron phosphate cathode material according to any one of the foregoing embodiments, comprising:

[0020] The pre-reaction liquid containing lithium source, iron source, phosphorus source, first titanium source, additives and inducer is subjected to hydrothermal reaction under hydrothermal reaction conditions to obtain the post-reaction liquid; the inducer includes at least one of citric acid, piperidine and isopropanol;

[0021] The solid phase in the liquid after the hydrothermal reaction is separated, and the solid phase is then subjected to acid washing, water washing, soaking in tetravalent titanium source and drying in sequence to obtain the material to be calcined;

[0022] The material to be calcined is subjected to reducing calcination and demagnetization in sequence to obtain the lithium iron phosphate cathode material.

[0023] In an optional embodiment, the molar ratio of lithium source, phosphorus source, iron source, inducer and first titanium source in the pre-reaction liquid is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1).

[0024] And / or, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate;

[0025] And / or, the additive is selected from ascorbic acid;

[0026] And / or, the additive is in the molar ratio of iron to iron in the iron source to 1.5%-2.5%;

[0027] And / or, the mass fraction of water in the pre-reaction liquid is 40%-60%.

[0028] In an optional embodiment, the hydrothermal reaction temperature is 160℃-200℃; the hydrothermal reaction time is 5h-7h.

[0029] And / or, the pH of the solution before the reaction is 6.0-8.0;

[0030] And / or, the pickling solution used is dilute sulfuric acid with a concentration of 0.005 mol / L to 0.1 mol / L.

[0031] In an optional embodiment, the tetravalent titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate.

[0032] And / or, the immersion solution used in the tetravalent titanium source immersion step is a tetravalent titanium source solution of 0.1 mol / L-0.2 mol / L, the mass ratio of solid phase to immersion solution in the tetravalent titanium source immersion step is 1:(1-3), and the immersion time is 8 min-12 min.

[0033] In an optional embodiment, the temperature of the reducing calcination step is 550℃-750℃, and the time is 2h-6h;

[0034] And / or, the atmosphere of the reducing calcination step includes an inert gas and carbon monoxide, wherein the volume fraction of carbon monoxide in the atmosphere is 1%-3%.

[0035] Thirdly, the present invention provides an electrode comprising the lithium iron phosphate cathode material described in any one of the foregoing embodiments.

[0036] Fourthly, the present invention provides a lithium-ion battery comprising the electrode sheet described in the foregoing embodiments.

[0037] The present invention has the following beneficial effects:

[0038] Increasing the corrosion resistance ω of lithium iron phosphate cathode materials is beneficial to improving cycle performance. However, if the corrosion resistance ω is too high, the crystal growth direction will change, resulting in a low lithium ion migration rate along the b-axis, which will lead to a decrease in rate performance. In this application, the corrosion resistance ω of lithium iron phosphate cathode materials is limited to a reasonable range, which is beneficial to balancing cycle performance and rate performance. Attached Figure Description

[0039] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 The image shows the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0042] This invention provides a lithium iron phosphate cathode material, wherein the corrosion resistance ω of the lithium iron phosphate cathode material is 0%-40%, and ω=(βI1) / (αI0), wherein,

[0043] α represents the mass fraction of iron (excluding ferrous iron) in the lithium iron phosphate cathode material.

[0044] β represents the mass fraction of elemental iron in the lithium iron phosphate cathode material;

[0045] I0 is the standard diffraction intensity of a specified crystal plane in a lithium iron phosphate crystal as recorded on a standard XRD card.

[0046] I1=Iˋ 最强 I1ˋ / I 最强 Among them, Iˋ 最强The strongest peak in the standard XRD pattern; I 最强 The XRD pattern of lithium iron phosphate cathode material with Iˋ 最强 The diffraction intensity of the corresponding crystal plane; I1ˋ is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of lithium iron phosphate cathode material;

[0047] The specified crystal plane is one of the (101), (112), (011), (010), and (001) crystal planes.

[0048] The applicant's research found that increasing the corrosion resistance ω of lithium iron phosphate cathode materials is beneficial to improving cycle performance. However, if the corrosion resistance ω is too high, the carbon coating layer will be too uniform and dense, which will lead to a decrease in rate performance. Therefore, in this application, the corrosion resistance ω of lithium iron phosphate cathode materials is limited to the range of 0%-40%, such as 0%, 4.4%, 8.9%, 13.3%, 17.8%, 22.2%, 26.7%, 31.1%, 35.6%, and 40%, which is beneficial to balancing cycle performance and rate performance.

[0049] In this application, the corrosion resistance strength ω is related to the exposed crystal planes of the lithium iron phosphate cathode material. Different exposed crystal planes of the lithium iron phosphate cathode material result in different densities and uniformities of the carbon coating layer covering its surface. Increasing the proportion of the specified crystal planes such as (101), (112), (011), (010), and (001) exposed by the lithium iron phosphate cathode material is beneficial to improving the density and uniformity of the carbon coating layer, thereby improving the cycle performance.

[0050] In an optional embodiment, the matrix is ​​lithium iron phosphate doped with titanium, which is beneficial for balancing cycle performance and rate performance.

[0051] In an optional implementation, α is 0%-1.5%, for example 0%, 0.17%, 0.33%, 0.5%, 0.67%, 0.83%, 1.0%, 1.17%, 1.33%, 1.5%;

[0052] And / or, β is 0%-0.2%, for example 0%, 0.02%, 0.04%, 0.06%, 0.08%, 0.10%, 0.12%, 0.14%, 0.16%, 0.18%, 0.2%.

[0053] In optional implementations, I1 / I0 is 1.2-3, such as 1.2, 1.3, 1.4, 1.5, 1.8, 2.0, 2.2, 2.4, 2.8, 3.0, or other values ​​within the range of 1.2-3.

[0054] The ferric ions or metallic iron present in lithium iron phosphate cathode materials are easily dissolved by hydrofluoric acid and migrate to the anode, where they are deposited on its surface. This electrochemical deposition can easily induce internal short circuits in the battery, so the amount of ferric ions or metallic iron should be minimized.

[0055] In an optional embodiment, the detection method of α includes: taking a lithium iron phosphate cathode material sample m0 = 5.0000g - 0.0010g and immersing it in 10ml of dilute sulfuric acid with a concentration of 0.01mol / L at 18-30℃ for 10min, and detecting the concentration of dissolved iron ions c0 g / ml by potentiometric titration, where α = (c0*10 / m0)*100%.

[0056] Lithium iron phosphate has a stable crystal structure, and the low hydrogen ion concentration in dilute sulfuric acid is insufficient to disrupt its crystal lattice; while trivalent iron compounds are mostly weak acid salts or oxides, which can gradually dissolve in dilute sulfuric acid to form soluble Fe. 3 + with sulfates; elemental iron is an active metal and can undergo a displacement reaction with dilute sulfuric acid, thereby dissolving elemental iron.

[0057] In an optional implementation, the detection method for β includes: taking a lithium iron phosphate cathode material sample m1 = 5.0000 g ± 0.0010 g and placing it in 10 ml of a 0.1 mol / L copper sulfate solution at 18-30 °C and reacting for 10 min, then using ICP to detect the iron ion concentration c1 g / ml in the solution, and β = (c1 * 10 / m1) * 100%.

[0058] Iron is an active metal that can displace copper in copper sulfate solution. The mass of iron in lithium iron phosphate cathode material can be calculated by measuring the amount of iron ions displaced, and then β can be calculated.

[0059] This invention also provides a method for preparing the lithium iron phosphate cathode material according to any one of the foregoing embodiments, comprising:

[0060] The pre-reaction liquid containing lithium source, iron source, phosphorus source, first titanium source, additives and inducer is subjected to hydrothermal reaction under hydrothermal reaction conditions to obtain the post-reaction liquid; the inducer includes at least one of citric acid, piperidine and isopropanol;

[0061] The solid phase in the liquid after the hydrothermal reaction is separated, and the solid phase is then subjected to acid washing, water washing, soaking in tetravalent titanium source and drying in sequence to obtain the material to be calcined;

[0062] The material to be calcined is subjected to reducing calcination and demagnetization in sequence to obtain the lithium iron phosphate cathode material.

[0063] In the preparation method of lithium iron phosphate cathode material of this application, because the activation energies of the growth of different crystal planes of lithium iron phosphate are inconsistent, during the hydrothermal process, the crystal plane with high activation energy usually grows preferentially and eventually closes, leaving the crystal plane with low activation energy exposed. In this application, the addition of an inducing agent can change the growth order of crystal planes with different activation energies, thereby exposing the specified crystal planes of the final lithium iron phosphate, thus affecting the uniformity of the carbon coating layer and obtaining lithium iron phosphate with a corrosion resistance strength ω within a reasonable range. Specifically, the -OH in isopropanol can prevent the rapid growth of high activation energy surfaces, eventually exposing the high activation energy surfaces; the N lone pair electrons in piperidine have a strong binding ability with nonpolar crystal planes, preventing the growth of high nonpolar crystal planes. By using different inducing agents, the purpose of inducing the growth or exposure of different crystal planes can be achieved.

[0064] Furthermore, in this application, the product is soaked in a tetravalent titanium source after the hydrothermal reaction, so that the tetravalent titanium source present on the surface of lithium iron phosphate can act as a protective agent during the subsequent reduction and calcination process, preventing the high-valent iron from being excessively reduced to elemental iron and avoiding the elemental iron content from being too high.

[0065] In an optional embodiment, the molar ratio of lithium source, phosphorus source, iron source, inducer, and first titanium source in the pre-reaction liquid is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1), for example (1.00:0.98:1.00:0.02:0.01), (1.01:0.982:1.01:0.03:0.02), (1.02:0.984:1.02:0.04:0.03), (1.03:0.986:1.03:0.05:0.04), (1. The inducing agents can also act as a carbon source, forming a carbon coating layer after calcination, thereby improving the cycle performance of the lithium iron phosphate cathode material. It should be noted that the lithium source in the pre-reaction solution can be lithium carbonate or lithium hydroxide, etc.

[0066] In an optional embodiment, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate.

[0067] In an optional embodiment, the additive is selected from ascorbic acid, which can prevent the oxidation of ferrous iron;

[0068] And / or, the additive is in the molar ratio of iron to iron in the iron source of 1.5%-2.5%, for example 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, or 2.5%.

[0069] Sufficient additives can reduce the oxidation of ferrous iron.

[0070] And / or, the temperature of the hydrothermal reaction is 160℃-200℃, for example 160℃, 164℃, 168℃, 172℃, 176℃, 180℃, 184℃, 188℃, 192℃, 196℃, 200℃; the hydrothermal reaction time is 5h-7h, for example 5h, 5.2h, 5.4h, 5.6h, 5.8h, 6.0h, 6.2h, 6.4h, 6.6h, 6.8h, 7h;

[0071] And / or, the pH of the pre-reaction solution is 6.0-8.0, for example 6.0, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, 8.0.

[0072] And / or, the mass fraction of water in the pre-reaction liquid is 40%-60%, for example 40%, 45%, 50%, 55%, 60%.

[0073] During the hydrothermal reaction, titanium phosphate precipitation solubility product is smaller than that of iron phosphate precipitation solubility product, which allows for the doping of titanium ions during the hydrothermal process.

[0074] In an optional embodiment, the washing solution used for pickling is dilute sulfuric acid with a concentration of 0.005 mol / L to 0.1 mol / L, to reduce impurities mixed in with lithium iron phosphate. The concentration of dilute sulfuric acid is, for example, 0.005 mol / L, 0.010 mol / L, 0.050 mol / L, 0.055 mol / L, 0.060 mol / L, 0.065 mol / L, 0.070 mol / L, 0.075 mol / L, 0.080 mol / L, 0.085 mol / L, 0.090 mol / L, 0.095 mol / L, or 0.100 mol / L.

[0075] In an optional embodiment, the tetravalent titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate.

[0076] And / or, the immersion solution used in the tetravalent titanium source immersion step is 0.1 mol / L-0.2 mol / L, for example, tetravalent titanium source solutions with concentrations of 0.10 mol / L, 0.11 mol / L, 0.12 mol / L, 0.13 mol / L, 0.14 mol / L, 0.15 mol / L, 0.16 mol / L, 0.17 mol / L, 0.18 mol / L, 0.19 mol / L, or 0.20 mol / L. The solid phase and immersion solution in the tetravalent titanium source immersion step... The mass ratio of the liquid is 1:(1-3), for example 1:1, 1:1.22, 1:1.44, 1:1.67, 1:1.89, 1:2.11, 1:2.33, 1:2.56, 1:2.78, 1:3; the soaking time is 8min-12min, for example 8min, 8.4min, 8.8min, 9.2min, 9.6min, 10min, 10.4min, 10.8min, 11.2min, 11.6min, 12min.

[0077] Introducing a sufficient amount of tetravalent titanium source is beneficial for reducing the elemental iron generated during the reducing calcination step. However, excessive tetravalent titanium source can lead to excessively high titanium doping levels, which is detrimental to both cycle performance and rate performance. Therefore, it is necessary to rationally set parameters such as the concentration of tetravalent titanium source in the immersion solution, the amount of immersion solution, and the immersion time.

[0078] In an optional embodiment, the temperature of the reducing calcination step is 550℃-750℃, for example, 550℃, 570℃, 590℃, 610℃, 630℃, 650℃, 670℃, 690℃, 710℃, 730℃, or 750℃; and the time is 2h-6h, for example, 2h, 2.4h, 2.8h, 3.2h, 3.6h, 4.0h, 4.4h, 4.8h, 5.2h, 5.6h, or 6h.

[0079] And / or, the atmosphere of the reducing calcination step includes an inert gas and carbon monoxide, wherein the volume fraction of carbon monoxide in the atmosphere is 1%-3%, for example 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, or 3.0%.

[0080] Reductive calcination can reduce ferric iron (Fe3+) in the precursor to ferrous iron (Fe2+), while inhibiting the oxidation of ferrous iron to ferric iron at high temperatures. This is beneficial for generating pure-phase lithium iron phosphate and reduces structural defects and electrochemical performance degradation caused by the presence of ferric iron.

[0081] This invention also provides an electrode sheet comprising the lithium iron phosphate cathode material described in any of the foregoing embodiments.

[0082] This invention also provides a lithium-ion battery, including the electrode sheet described in the foregoing embodiments.

[0083] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0084] Example 1

[0085] This embodiment provides a method for preparing lithium iron phosphate cathode material, specifically including the following steps:

[0086] Nitrogen gas was introduced into a hydrothermal reactor containing deionized water to remove oxygen from the water. Lithium hydroxide, phosphoric acid, ferrous sulfate, and titanium oxalate were weighed in a molar ratio of 1.05:1:1.02:0.02 and added to the hydrothermal reactor. Ascorbic acid, at a molar amount equal to 2% of the iron content in ferrous sulfate, was then weighed. Citric acid, at a molar amount equal to 3% of the ferrous sulfate content, was added to the reactor, and the pH was adjusted to 7. The resulting pre-reaction solution contained 50% water was obtained.

[0087] The temperature in the hydrothermal reactor was adjusted to 180℃ and hydrothermally heated for 6 hours. After the hydrothermal reaction was completed, the temperature was lowered and the liquid after the hydrothermal reaction was separated into solid and liquid phases. The solid phase was washed with 0.01mol / L dilute sulfuric acid and then washed with deionized water. Next, it was soaked in a 0.2mol / L tetrabutyl titanate aqueous solution, wherein the mass ratio of the washed solid phase to the tetrabutyl titanate aqueous solution was 1:2 and the soaking time was 10min. After soaking, the solid phase was separated and vacuum dried to obtain the material to be calcined.

[0088] The material to be calcined was subjected to reducing calcination in a nitrogen / carbon monoxide mixed atmosphere. The volume fraction of carbon monoxide in the mixed atmosphere was 2%. The calcination temperature was 700℃, and the calcination time was 4 hours. After calcination, the material was cooled and demagnetized to obtain lithium iron phosphate cathode material. The XRD pattern is shown below. Figure 1 As shown.

[0089] Example 2

[0090] This embodiment provides a method for preparing lithium iron phosphate cathode material, specifically including the following steps:

[0091] Nitrogen gas was introduced into the hydrothermal reactor to remove oxygen from the deionized water. Lithium hydroxide, phosphoric acid, ferrous chloride, and titanium sulfate were weighed in a molar ratio of 1.05:1:1.02:0.02 and added to the hydrothermal reactor. Ascorbic acid, at a molar amount equal to 2% of the iron content in ferrous chloride, was then weighed. Piperidine, an inducing agent, at a molar amount equal to 3% of the ferrous chloride content, was added to the reactor, and the pH was adjusted to 7. The resulting pre-reaction solution contained 50% water by mass was obtained.

[0092] The temperature in the hydrothermal reactor was adjusted to 180℃ and hydrothermally heated for 6 hours. After the hydrothermal reaction was completed, the temperature was lowered and the liquid after the hydrothermal reaction was separated into solid and liquid phases. The solid phase was washed with 0.01mol / L dilute sulfuric acid and then washed with deionized water. Next, it was soaked in 0.1mol / L tetraethyl titanate aqueous solution, where the mass ratio of the water-washed solid phase to the tetraethyl titanate aqueous solution was 1:1 and the soaking time was 10 minutes. After soaking, the solid phase was separated and vacuum dried to obtain the material to be calcined.

[0093] The material to be calcined was placed in a nitrogen / carbon monoxide mixed atmosphere for reducing calcination. The volume fraction of carbon monoxide in the mixed atmosphere was 1%, the calcination temperature was 650℃, and the calcination time was 6h. After calcination, the material was cooled and demagnetized to obtain lithium iron phosphate cathode material.

[0094] Example 3

[0095] This embodiment provides a method for preparing lithium iron phosphate cathode material, specifically including the following steps:

[0096] Nitrogen gas was introduced into the hydrothermal reactor to remove oxygen from the deionized water. Lithium hydroxide, phosphoric acid, ferrous oxalate, and titanium sulfate were weighed in a molar ratio of 1.05:0.98:1.0:0.01 and added to the hydrothermal reactor. Ascorbic acid, weighed at 2% of the molar amount of iron in ferrous oxalate, was added. Citric acid, weighed at 2% of the molar amount of ferrous oxalate as an inducer, was added to the reactor, and the pH was adjusted to 7 to obtain a pre-reaction solution with a water mass fraction of 50%.

[0097] The temperature in the hydrothermal reactor was adjusted to 180℃ and hydrothermally heated for 6 hours. After the hydrothermal reaction was completed, the temperature was lowered and the liquid after the hydrothermal reaction was separated into solid and liquid phases. The solid phase was washed with 0.01mol / L dilute sulfuric acid and then washed with deionized water. Next, it was soaked in a 0.2mol / L tetrabutyl titanate aqueous solution, wherein the mass ratio of the washed solid phase to the tetrabutyl titanate aqueous solution was 1:3 and the soaking time was 10 minutes. After soaking, the solid phase was separated and vacuum dried to obtain the material to be calcined.

[0098] The material to be calcined was placed in a nitrogen / carbon monoxide mixed atmosphere for reducing calcination. The volume fraction of carbon monoxide in the mixed atmosphere was 3%, the calcination temperature was 700℃, and the calcination time was 2h. After calcination, the material was cooled and demagnetized to obtain lithium iron phosphate cathode material.

[0099] Example 4

[0100] This embodiment provides a method for preparing lithium iron phosphate cathode material, specifically including the following steps:

[0101] Nitrogen gas was introduced into the hydrothermal reactor to remove oxygen from the deionized water. Lithium hydroxide, phosphoric acid, ferrous sulfate, and titanium oxalate were weighed in a molar ratio of 1.05:0.98:1.0:0.1 and added to the hydrothermal reactor. Ascorbic acid, at a molar amount equal to 2% of the iron content in the ferrous sulfate, was then weighed. Piperidine, an inducing agent, at a molar amount equal to 5% of the ferrous sulfate, was added to the reactor, and the pH was adjusted to 7 to obtain a pre-reaction solution with a water mass fraction of 50%.

[0102] The temperature in the hydrothermal reactor was adjusted to 180℃ and hydrothermally heated for 6 hours. After the hydrothermal reaction was completed, the temperature was lowered and the liquid after the hydrothermal reaction was separated into solid and liquid phases. The solid phase was washed with 0.01mol / L dilute sulfuric acid and then washed with deionized water. Then it was soaked in 0.2mol / L tetrabutyl titanate aqueous solution, wherein the mass ratio of the washed solid phase to the tetrabutyl titanate aqueous solution was 1:1 and the soaking time was 10min. After soaking, the solid phase was separated and vacuum dried to obtain the material to be calcined.

[0103] The material to be calcined was placed in a nitrogen / carbon monoxide mixed atmosphere for reducing calcination. The volume fraction of carbon monoxide in the mixed atmosphere was 1%, the calcination temperature was 700℃, and the calcination time was 2h. After calcination, the material was cooled and demagnetized to obtain lithium iron phosphate cathode material.

[0104] Example 5

[0105] This embodiment provides a method for preparing lithium iron phosphate cathode material. The main difference from Embodiment 1 is that 5% of the amount of ferrous sulfate and citric acid as an inducer are added to the reaction vessel, resulting in ω being within a reasonable range, but I1 / I0 being too low.

[0106] Example 6

[0107] This embodiment provides a method for preparing lithium iron phosphate cathode material. The main difference from Example 1 is that the mass ratio of the solid phase to the tetrabutyl titanate aqueous solution is 1:1, and the soaking time is 10 min, which leads to an increase in α.

[0108] Example 7

[0109] This embodiment provides a method for preparing lithium iron phosphate cathode material. The main difference from Embodiment 1 is that the volume fraction of carbon monoxide in the mixed atmosphere is 5%, which leads to an increase in β.

[0110] Comparative Example 1

[0111] This comparative example provides a method for preparing lithium iron phosphate cathode material. The main difference from Example 4 is that the first titanium source and the tetravalent titanium source are not introduced, and the inducing agent is replaced with a carbon source. The specific steps include:

[0112] Nitrogen gas was introduced into the hydrothermal reactor to remove oxygen from the deionized water. Lithium hydroxide, phosphoric acid, and ferrous sulfate were weighed in a molar ratio of 1.05:0.98:1.0 and added to the hydrothermal reactor. Then, ascorbic acid was weighed in at a molar amount equal to 2% of the iron content in the ferrous sulfate. Glucose was added at a molar amount equal to 5% of the ferrous sulfate as a carbon source, and the pH was adjusted to 7 to obtain a pre-reaction solution with a water content of 50%.

[0113] The temperature in the hydrothermal reactor was adjusted to 180℃ and hydrothermally heated for 6 hours. After the hydrothermal reaction was completed, the temperature was lowered and the liquid after the hydrothermal reaction was separated into solid and liquid phases. The solid phase was washed with dilute sulfuric acid with a concentration of 0.01mol / L, then washed with deionized water and vacuum dried to obtain the material to be calcined.

[0114] The material to be calcined was heated and calcined in a nitrogen-filled atmosphere. The volume fraction of carbon monoxide in the mixed atmosphere was 2%, the calcination temperature was 700℃, and the calcination time was 2h. After calcination, the material was cooled and demagnetized to obtain lithium iron phosphate cathode material.

[0115] Comparative Example 2

[0116] This comparative example provides a method for preparing lithium iron phosphate cathode material. The main difference from Example 1 is that the first titanium source is replaced with an equal mass of zirconium sulfate, resulting in a higher corrosion resistance ω.

[0117] Comparative Example 3

[0118] This comparative example provides a method for preparing lithium iron phosphate cathode material. The main difference from Example 1 is that tetrabutyl titanate is replaced with an equal mass of tetraethoxyzirconium, resulting in an excessively high corrosion resistance ω.

[0119] Test Example 1:

[0120] Some properties of the lithium iron phosphate cathode materials prepared in the above embodiments and comparative examples were tested, and the test results are shown in Table 1. The test methods are as follows:

[0121] The detection method for α includes: taking a lithium iron phosphate cathode material sample m0 = 5.0000 g ± 0.0010 g and immersing it in 10 ml of dilute sulfuric acid with a concentration of 0.01 mol / L at 18-30℃ for 10 min, and detecting the concentration of dissolved iron ions c0 g / ml by potentiometric titration, α = (c0 * 10 / m0) * 100%;

[0122] The detection method for β includes: taking a lithium iron phosphate cathode material sample m1 = 5.0000g ± 0.0010g and placing it in 10ml of copper sulfate solution with a concentration of 0.1mol / L at 18-30℃ and reacting for 10min, and then using ICP to detect the iron ion concentration c1g / ml in the solution, β = (c1*10 / m1)*100%.

[0123] In Table 1, the corrosion resistance strength ω is calculated based on the maximum values ​​of I1 / I0 corresponding to the (101), (112), (011), (010), and (001) crystal planes.

[0124] I0 is the standard diffraction intensity of a specified crystal plane in a lithium iron phosphate crystal as recorded on a standard XRD card.

[0125] I1=I` 最强 I1` / I 最强 Among them, I` 最强 The strongest peak in the standard XRD pattern; I 最强 The XRD pattern of lithium iron phosphate cathode material is related to I` 最强 The diffraction intensity of the corresponding crystal plane; I1` is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of lithium iron phosphate cathode material.

[0126] Test Example 2:

[0127] The lithium iron phosphate cathode materials prepared in the above embodiments and comparative examples were used to prepare electrode sheets, which were then assembled into batteries. The specific method included: mixing lithium iron phosphate, conductive carbon black, and PVDF in a mass ratio of 8:1:1 and adding them to NMP solvent for magnetic stirring to form a uniform slurry; coating the slurry onto aluminum foil and vacuum drying at 100°C for 12 hours; cutting the prepared electrode sheets into 12mm circular pieces using a slicing machine and weighing them; controlling the load of each electrode at 1.5 mg / cm². 2 -3mg / cm 2 This electrode sheet was used to assemble a coin cell in a glove phase, with lithium metal as the negative electrode, Celgrad2400 as the separator, and a 1 mol / L LiPF6 organic solution (the solvent was prepared by mixing EC, DEC, and DMC in a volume ratio of EC:DEC:DMC = 1:1:1) as the electrolyte to assemble a CR2032 coin cell.

[0128] Performance testing method: Constant current charge and discharge tests were conducted using a Blue Battery test cabinet.

[0129] Table 1

[0130]

[0131] Note: The crystal planes specified in the table are the crystal planes corresponding to I1 / I0 used to calculate the corrosion resistance strength ω.

[0132] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material comprises a matrix and a carbon coating layer covering the matrix. The corrosion resistance ω of the lithium iron phosphate cathode material is 8%-40%, and ω=(βI1) / (αI0), where... α represents the mass fraction of iron (excluding ferrous iron) in the lithium iron phosphate cathode material. β represents the mass fraction of elemental iron in the lithium iron phosphate cathode material; I0 is the standard diffraction intensity of a specified crystal plane in a lithium iron phosphate crystal as recorded on a standard XRD card. I1=I` 最强 I1` / I 最强 Among them, I` 最强 The strongest peak in the standard XRD pattern; I 最强 The XRD pattern of lithium iron phosphate cathode material is related to I` 最强 The diffraction intensity of the corresponding crystal plane; I1` is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of lithium iron phosphate cathode material; The specified crystal plane is one of the (101), (112), (011), (010), and (001) crystal planes.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The matrix is ​​lithium iron phosphate doped with titanium; And / or, α is 0%-1.5%; And / or, β is 0%-0.2%; And / or, I1 / I0 is 1.2-3.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The detection method for α includes: taking a lithium iron phosphate cathode material sample m0 = 5.0000 g ± 0.0010 g and immersing it in 10 ml of dilute sulfuric acid with a concentration of 0.01 mol / L at 18℃-30℃ for 10 min, and detecting the concentration of dissolved iron ions c0 g / ml by potentiometric titration, α = (c0*10 / m0)*100%; And / or, the detection method for β includes: taking a lithium iron phosphate cathode material sample m1 = 5.0000 g ± 0.0010 g and placing it in 10 ml of copper sulfate solution with a concentration of 0.1 mol / L at 18-30℃ and reacting for 10 min, and then using ICP to detect the iron ion concentration c1 g / ml in the solution, β = (c1*10 / m1)*100%.

4. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, include: The pre-reaction liquid containing lithium source, iron source, phosphorus source, first titanium source, additives and inducer is subjected to hydrothermal reaction under hydrothermal reaction conditions to obtain the post-reaction liquid; the inducer includes at least one of citric acid, piperidine and isopropanol; The solid phase in the liquid after the hydrothermal reaction is separated, and the solid phase is then subjected to acid washing, water washing, soaking in tetravalent titanium source and drying in sequence to obtain the material to be calcined; The material to be calcined is subjected to reducing calcination and demagnetization in sequence to obtain the lithium iron phosphate cathode material.

5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The molar ratio of lithium source, phosphorus source, iron source, inducer, and first titanium source in the pre-reaction liquid is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1). And / or, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate; And / or, the additive is selected from ascorbic acid; And / or, the molar ratio of the additive to the iron in the iron source is 1.5%-2.5%; And / or, the mass fraction of water in the pre-reaction liquid is 40%-60%.

6. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The hydrothermal reaction temperature is 160℃-200℃, and the hydrothermal reaction time is 5h-7h; And / or, the pH of the solution before the reaction is 6.0-8.0; And / or, the pickling solution used is dilute sulfuric acid with a concentration of 0.005 mol / L to 0.1 mol / L.

7. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The tetravalent titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate. And / or, the immersion solution used in the tetravalent titanium source immersion step is a tetravalent titanium source solution of 0.1 mol / L-0.2 mol / L, and the mass ratio of solid phase to immersion solution in the tetravalent titanium source immersion step is 1:(1-3), and the immersion time is 8 min-12 min.

8. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The temperature for the reducing calcination step is 550℃-750℃, and the time is 2h-6h. And / or, the atmosphere of the reducing calcination step includes an inert gas and carbon monoxide, wherein the volume fraction of carbon monoxide in the atmosphere is 1%-3%.

9. An electrode sheet, characterized in that, Including the lithium iron phosphate cathode material as described in any one of claims 1-3.

10. A lithium-ion battery, characterized in that, Includes the electrode sheet as described in claim 9.

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